The studies of the human erythrocyte membrane involve 1) identification of the membrane components of the Mg 2 ion-dependent, the (Na+K)-dependent and the Ca 2 ion-stimulated ATPase 2) the identification of the membrane components of the "facilitated diffusion" system for glucose 3) the identification of the RhD antigen. These objectives will be accomplished through the use of various protein modifying agents, such as, N-acetylimidazole, a variety of dione compounds, tetranitromethane, RhD antibody, proteolytic and lipolytic agents. It is anticipated that these agents, through their specificity, will help identify the proteins located at the binding sites of the various ligands. In addition, extensive studies on the role of cholesterol in the membrane in general, and specifically its involvement in each of the ATPase systems. These studies will include the partial substitution of various cholesterol derivatives, such as, stigmasterol, ergosterol, 5 alpha-cholestan-3-one, etc., to identify structural characteristics of the molecule necessary for its proper incorporation into the erythrocyte membrane.